Field of the Invention
The present invention relates to an image reading apparatus configured to read an image of an original (original image) while conveying the original.
Description of the Related Art
Some image forming apparatus, for example, a copying machine and a facsimile machine, include an image reading apparatus for reading an original image. The image reading apparatus can continuously read respective images of a plurality of originals through use of an auto document feeder (ADF). When the ADF is used, the original is fed from an original tray, and conveyed to a reading position by a conveyance roller pair. The conveyance roller pair is formed so as to sandwich a conveyance path with a pair of conveyance rollers, and is configured to rotate while sandwiching an original, to thereby convey the original. Skew may occur in the original being conveyed by the conveyance roller pair due to variations in how the respective rollers that form the conveyance roller pair are parallel to a direction (main scanning direction) perpendicular to a conveyance direction of the original. Due to the skew of the original, the read original image become tilted. When the original image becomes tilted, there occurs such an image defect that the original image is partially missing in the read image or a larger portion of a background part (conveyance path) of the original is included.
There is an image reading apparatus configured to correct a tilt of the original image by image processing when such an image defect occurs. The image reading apparatus extracts a boundary between an original part and the background part of the read original image, to thereby detect an angle (skew amount) with respect to a conveyance direction of the original. The image reading apparatus corrects the tilt of the original image by the image processing based on the detected skew amount. Such correction of the tilt of the original image using the image processing has a limit to the amount that can be corrected. For example, such correction of the tilt of the original image cannot be conducted for the original being skew-fed too considerably to read the entirety of the original image corresponding to one sheet. Therefore, an image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2012-244383 temporarily finishes reading processing by determining that the reading of the original image has failed when the tilt of the original image cannot be corrected by the image processing due to considerable skew. At a time of restarting a reading processing operation, the image reading apparatus mechanically conducts the skew correction for the original before the original reaches the reading position. When the skew amount of the original abruptly becomes larger, the configuration of the image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2012-244383 is effective.
The mechanical skew correction for the original is conducted by bringing a leading edge of the original in the conveyance direction into abutment with the registration roller pair provided on the conveyance path. In the mechanical skew correction, collision noise is produced when the original is brought into abutment with the registration roller pair, or driving noise is produced when the registration roller pair is driven or stopped. Therefore, the image reading apparatus capable of the tilt correction for the original image using the image processing and the mechanical skew correction prioritizes the tilt correction for the original image using the image processing in order to reduce noise.
There is a case where the original is liable to be skew-fed with a fixed angle depending on a state where the original is placed on the original tray or a relationship between widths (lengths in a direction perpendicular to the conveyance direction) of the conveyance roller pair and the original. In such a case, the image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2012-244383 forcedly finishes the reading processing when an original being skew-fed with a skew amount exceeding a limit of an amount of the tilt correction for the original image using the image processing appears before the image reading apparatus finishes reading a plurality of originals placed on the original tray. When the reading processing is forcedly finished, the plurality of originals fail to be read normally, which lowers usability. For example, when the plurality of originals placed on the original tray are liable to be skew-fed, the image reading apparatus corrects the tilt of the original image by the image processing, and temporarily finishes reading the original when the original image exceeding the limit of the amount of the correction is read. The image reading apparatus mechanically corrects the skew of the original after the original is set again. To that end, a user needs to perform twice an operation for setting the original on the original tray, resulting in poor operability. Therefore, the present invention has a main object to provide an image reading apparatus configured to efficiently correct a defect of an original image due to the skew of an original.